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there is just difference in layout and quiet diffs in funcionallity it just depends on what you are familliar with. if you comming from windows world, you will find maybe KUbuntu little easier to work with for example, like zorin os has built in windows, unity, mac... layouts.
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Dimitris TheodoridisMar 27 '13 at 13:20

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There are thousands, not only the DM, but several more.
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BraiamMar 11 '14 at 16:34

4 Answers
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Ubuntu is the original creation for better or worse. It aims to be a 'modern' desktop system with a lot of eye candy. For older systems, it probably is not the best system as it is somewhat heavy on your system, although most likely not as heavy as Windows. Canonical (the company that is the creator of Ubuntu) has taken a lot of flak lately for going more their own way instead of staying with the community that helped develop Ubuntu. Unlike Microsoft or Apple products, Linux is heavily community-based. While there are some people who get paid to work on Linux, the overwhelming majority do this out of their own good will. Ubuntu was originally based on the Gnome desktop enviros. Canonical has abandoned those in favor or their own original desktop system. There was some effort made to retain a Gnome-LIKE environment alternative, but I am unsure if Canonical has abandoned that for later versions.

In the past, as in all things Linux, some people decided they wanted to make a different version of Ubuntu. One version is based on the KDE desktop environment. It is like how different version of Windows look different, but it would be more akin to how Apple looks and feels different. One thing about KDE is there are a lot of options. For your average user it may be overwhelming. If you like lots of things to play with, then Kubuntu is a nice choice. Personally, I like Kubuntu. There are a lot of applications included designed to fit with the style and theme of Kubuntu. If you prefer a more traditional menu-style system, Kubuntu would be better although you are not limited to using it that way.

Lubuntu is a nice alternative. It is based on the LXDE environment which aims to be lightweight. It's a great disto (short for distribution) for old systems (not ancient systems) as it aims to keep the impact on your system low. However, some people just like Lubuntu on their newer systems because it runs fast and takes up less resources. I'm not sure if there are any apps specifically designed for Lubuntu, but the apps that are included are picked because they are lightweight too. They won't have all the bells and whistles of apps on vanilla Ubuntu or on Kubuntu, but neither are they stripped. (Most users only use a small percent of all the features in a given program, so the programs focus on the most common used items.)

SIDENOTE: A new disto called LXLE based on Lubuntu was recently announced:
"LXLE, which is short for “Lubuntu Extra Life Extension,” is actually based on Lubuntu 12.04, but it adds many extra features. First and foremost, LXLE is designed “to try to remain as light as possible while providing an incredibly rich environment of programs, features, and aesthetics,” the project team explains."
http://www.pcworld.com/article/2031825/have-an-older-pc-try-the-new-ubuntu-linux-based-lxle.html
-- If might be worth looking at if Lubuntu interests you, but you may want to stick with Lubuntu if you looking for something that already has been established for a whiles...

It's a lot personal opinion and/or preference. Underneath, both Kubuntu and Lubuntu are based off of Ubuntu's underworkings. Strictly speaking, the changes are more than just the desktop environments, but for practical purposes that's the easiest way to view it.

You might also look at Linux Mint which is based off of Ubuntu too. It's grown from a small but popular alternative to Ubuntu to become somewhat of a rival project spawning over different desktop enviros. Right now it is based over a combination of Cinnamon (a fork, similar to how Kubuntu and Lubuntu are forks of Ubuntu, of Gnome 3) and Mate (which is what Gnome 2 has changed to). Linux Mint was designed to provide a complete experience right out of the box without having to download additional packages after installing the OS. Personally I found it lighter weight than Ubuntu in the past and a crisper interface. It's a nice fit between Ubuntu and Lubuntu. They also have a (relatively) new KDE version of Mint.

Just in case you are wondering, desktop environments are not limited to Ubuntu. They tend to be developed independently of the OS so they can fit into different Linux distributions. And yes, you can actually install multiple desktop environments onto the same installation, but things can start looking quite messy (as each environment has their own particular set of applications which may not be on the others) although generally everything runs okay. Aside from being a mess, you get to pick at login which desktop to use which can be kind of fun if not necessarily the best experience.

For Ubuntu, Kubuntu, and Lubuntu I recommend going with 12.04. That is a long-term support release and will give you more time to get use to them. Mint follows their own numbering scheme. Mint 14 is the latest version.

That's probably a lot of info, and that's just an overview. As others said, the best way is to just try each one out. If you have a spare machine, you can create one of more environments and install a disto of your choice. After installing, you'll get a menu when you boot up to boot to Windows or Linux. Have fun! =)

There are significant, non-trivial differences. If your interest is not merely theoretical, the best way for you to experience the differences on your PC(s) would be to run each flavor separately from a Live CD or USB.

Kubuntu is based on KDE desktop, while Ubuntu uses the GNOME desktop. Gnome requires a lot more typing of commands in the terminal than KDE, which is more of a user-friendly environment. KDE is glossier, slicker, and can have things like 3D bouncing/stretching/exploding folders, while Gnome is more scaled back, plain jane. Gnome is "supposed" to be better on older equipment, using less resources, but I find Kubuntu so much faster than Ubuntu it's incredible.